Hugo Ricardo Zschommler Sandim

Nucleation and growth during recrystallization

The evolution in the understanding of the recrystallization phenomena is summarized in this paper. Initially the main developments concerning recrystallization are presented from a historical perspective. Definitions and concepts involving recrystallization are presented regarding it as a solid-state reaction that occurs by nucleation and growth. The recrystallization nucleation mechanisms are subsequently discussed. Finally, the growth step is highlighted, emphasizing boundary and sub-boundary mobilities and the forces acting on the high angle grain boundaries that sweep the microstructure during recrystallization.

Kinetics of thermal decomposition of titanium hydride powder using in situ high-temperature X-ray diffraction (HTXRD)

The thermal decomposition of titanium hydride powder (d-phase) to titanium (a-phase) was investigated by means of thermogravimetric analysis (TGA) and high-temperature X-ray diffraction (HTXRD) in high vacuum. The d-to-a phase transformation was followed in situ by HTXRD at temperatures varying from room temperature up to 1000 °C. The transformation was also analyzed as a function of time at isothermal conditions from 450 to 650 °C. The results of TGA show that the decomposition of the titanium hydride becomes significant at about 450 °C. Above 500 °C the decomposition is completed in times shorter than 50 minutes. The apparent activation energy for hydrogen desorption was found to be 63 ± 6 kJ.mol-1.

Annealing effects on the magnetic properties of a multifilamentary Cu–Nb composite

In this paper, the annealing effects on the magnetic properties of a multifilamentary Cu–15vol%Nb composite were investigated. During vacuum annealing, noticeable changes take place in the microstructure, mostly the partial spheroidization and further coarsening of the niobium filaments. Results show that spheroidization becomes noticeable at about 700 °C and, even after annealing at severe conditions, e.g. 1050 °C for 32 h, the continuity of the niobium-conducting path is partially preserved. The influence of these microstructural changes on the magnetic properties of the multifilamentary Cu–Nb composite conductor is discussed.

Orientation effects during grain subdivision and subsequent annealing in coarse-grained tantalum

Abstract Noticeable orientation effects were observed in two neighboring grains in tantalum during cold rolling and further annealing. The stored energy varied markedly from grain to grain and, consequently, their response to annealing: grain A recrystallized in full extent while grain B was strongly softened by static recovery.

Grain subdivision and recrystallization in oligocrystalline tantalum during cold swaging and subsequent annealing

A coarse-grained ingot of high-purity tantalum was deformed by swaging at room temperature to a strain of 1.28. During annealing at 900°C for 30 min two neighboring grains were observed to behave quite differently. Electron backscattering diffraction (EBSD) results show noticeable differences in terms of the misorientations developed in both grains. The grain developing larger misorientations recrystallized much more readily than the other. The result is interpreted in terms of the differences in grain subdivision into strongly misoriented regions.

Recrystallization behavior of a cold-rolled niobium bicrystal

A high-purity coarse-grained niobium bicrystal was 70% cold rolled in multiple passes. Deformation occurred in an inhomogeneous manner in both grains giving rise to a banded structure. In consequence, highly misoriented boundaries were developed in the microstructure in a wide range of misorientations, many reaching about 55°. These boundaries act as effective nucleation sites for recrystallization. The microstructure of both grains was investigated using electron backscatter diffraction (EBSD) in the cold-worked state in order to quantify the misorientations (ψ) associated to these bands. Upon annealing at 800 and 900 °C, the new recrystallized grains were nucleated preferentially at deformation heterogeneities and in the vicinity of the prior grain boundary in this bicrystal.

The effect of excess Al and fabrication environment on the composition and microstructure of V–Al alloys

Abstract V–Al alloys have been used commercially to produce pure vanadium by electron beam melting (EBM), as well as to obtain Ti-based alloys (e.g., Ti–6Al–4V). Concerning the production of pure vanadium, earlier studies have indicated that V–Al alloys presenting Al contents in the range of 10–15 wt% are the most suitable for direct EBM. In this study, we have investigated the aluminothermic reduction (ATR) of V 2 O 5 in order to define process parameters for the production of V–Al alloys with composition in the previously mentioned range. The effect of excess Al and reactor atmosphere (air, argon) on the alloy composition (Al, O, N) and microstructure was studied. The produced alloys were characterized by chemical analysis (Al, O, N), scanning electron microscopy (SEM) (back-scattered electron image, BSEI), electron probe microanalysis (EPMA) (wavelength dispersive spectroscopy, WDX), X-ray diffraction (XRD) and microhardness measurements.

An EBSD Investigation on Deformation-Induced Shear Bands in Ti-Bearing IF-Steel under Controlled Shock-Loading Conditions

We report the results of the microstructural characterization of a Ti-bearing IF-steel deformed at high strain rates (» 6.104 s-1) in a split Hopkinson bar. The shock-loading tests were performed in hat-shaped specimens to induce the formation of adiabatic shear bands (ASB). The samples were deformed at 223 K and 298 K. High-resolution electron backscatter diffraction (EBSD) reveals the development of an ultrafine-grained structure within the ASB. A closer inspection reveals the presence of deformation twins in grains adjacent to the shear band. These twins bend towards the ASB suggesting that mechanical twinning occurs before the flow associated to shear banding. The results of microtexture have indicated the presence of a sharp <111> g-fiber texture in the ASB for both temperatures.

Annealing effects on the electrical and superconducting properties of a Cu-15vol%Nb composite conductor

In this paper, annealing effects on the electrical and superconducting properties of a multifilamentary Cu-Nb composite were investigated. During annealing, noticeable changes in the microstructure take place, in special those related to recovery and recrystallization of copper and niobium followed by spheroidization and further coarsening of the Nb-filaments. Results show that spheroidization becomes significant only above 800/spl deg/C and, even after annealing at severe conditions, e.g., 1050/spl deg/C for 32 h, the continuity of the niobium-conducting path is partially preserved. The influence of these microstructural changes on the superconducting and normal properties of the multifilamentary Cu-15vol%Nb composite conductor is discussed.

Three-Dimensional Characterization of Pores in Ti-6Al-4V Alloy

The direct three-dimensional characterization of opaque materials through serial sectioning makes possible to visualize and better quantify a material microstructure, using classical metallographic techniques coupled with computer-aided reconstruction. Titanium alloys are used as biomaterials for bone implants because of its excellent mechanical properties, biocompatibility and enhanced corrosion resistance. The Ti-6Al-4V alloy (in wt. (%)) with porous microstructure permits the ingrowths of new-bone tissues improving the fixation bone/implant. This is important to understand connectivity, morphology and spatial distribution of pores in microstructure. The Ti-6Al-4V alloy compacts were produced by powder metallurgy and sintered at three distinct temperatures (1250, 1400 and 1500 °C) to obtain distinct microstructures in terms of residual porosity. The visualization of the reconstructed 3D microstructure provides a qualitative and quantitative analysis of the porosity of Ti6Al4V alloy (volume fraction and pore morphology).